Conventionally, to prevent the collapses of exposed terrain surfaces such as a face of slope and a tunnel excavation, there have been used spraying processes which spray an accelerated mortar or concrete on the spray surface, in which process an accelerator is added to cement composition such as mortar or concrete. Generally, as the accelerator, alkali metal salts (sodium and potassium) of aluminate, silicic acid, carbonic acid, etc. have been widely used. Since these compounds have strong alkalinity, they have adverse effects on the environment and the human body in spraying operation; for example, they may attack the skin and cause eye inflammation or corneal corrosion thereby impairing visual acuity.
Moreover, alkaline accelerators, which contains alkali metals such as sodium or potassium, may pose a risk of inducing alkali-aggregate reaction with the aggregates in concrete and, therefore, are not preferable in the viewpoint of durability of the concrete.
Moreover, sprayed concrete generally includes a large quantity of cement per unit area, and therefore a large quantity of alkali (sodium and potassium) is also induced from the cement. Further, since the ratio of fine aggregates is also large, there was a high probability of causing alkali-aggregate reaction. Thus, when a reactive aggregate was used for sprayed concrete, there was a high risk of causing cracks in the concrete after cure thereby impairing the durability. As the countermeasure of this, attempts have been made to use aggregates which would not contribute to alkali-aggregate reaction or to use low alkali type cement; however, it was difficult to fully prevent the problem.
Under these current circumstances, acidic liquid accelerators composed of aluminum sulfate, aluminum hydroxide, and the fluorides of aluminum have been proposed.
For example, a concrete accelerator consisting of aluminum fluoride, acidic or basic solution of aluminum containing aqueous hydrofluoric acid solution of aluminum, lithium silicate, and lithium aluminate has been proposed (see, for example, patent document 1). Moreover, an accelerator which consists of nitrate, sulfate, glycolate, lactate, etc. of aluminum, a compound forming agent, and a corrosion inhibitor and which contains no alkalis and chlorides has been proposed (see, for example, patent document 2).
Furthermore, an accelerator that contains aluminum salts, which contains aqueous fluorides, and sulfates, and that does not contain alkali metals and chlorides has been proposed (see, for example, document 3). Furthermore, a liquid accelerator containing aluminum, sulfur, sodium, and fluorine has been proposed (see, for example, patent document 4). There has been further proposed an accelerator that consists of a fluoride-containing aqueous aluminum salt, which is obtained through the reaction between the aluminum hydroxides or aluminum oxides and hydrofluoric acid, and a sulfate-containing aqueous aluminum salt such as aluminum sulfate and basic aluminum sulfate, and that does not contain alkali metals and chlorides (see, for example, patent document 5).
However, the above described technologies have not provided a liquid accelerator having a sufficient accelerating effect. The present inventors has focused attention on the effects of alkali metals for improving the initial strength development of concrete, and proposed, in Japanese Patent Application No. 2003-62114, a liquid accelerator that contains aluminum sulfate, alkanolamine, alkylene diamine, hydrofluoric acid, and 1 to 8.5% by total alkali of alkali metal salts such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium sulfate, potassium sulfate. However, although this liquid accelerator excelled in accelerating effect and initial strength development, it had a problem in solution stability at low temperatures.    [Patent document 1] Japanese Patent Laid-Open No. 2001-130935.    [Patent document 2] Japanese Patent Laid-Open No. 2001-180994, U.S. Pat. No. 6,514,327    [Patent document 3] Japanese Patent Laid-Open No. 2002-29801.    [Patent document 4] Japanese Patent Laid-Open No. 2002-47048.    [Patent document 5] Japanese Patent Laid-Open No. 2002-80250.